1. Field of the Invention
The invention relates to a method for producing an optical unit having an optical element deviating from a circular shape, for an optical imaging system, in particular for a projection objective in semiconductor lithography in the EUV region.
2. Description of the Related Art
The invention also relates to an optical unit having an optical element for an optical imaging system.
Projection objectives in semiconductor lithography for producing semiconductor elements which operate in the EUV region at wavelengths of around 11 to 13 nm are subject to extremely high requirements placed on the optical elements. This applies, for example, firstly to the useful optical surface within a particular specification (fitting, roughness) with reference to the production and, secondly, also to a later adjustment with reference to a very exact installation position in an objective. This relates, in particular, to mirrors which are required instead of lenses given the very short wavelengths mentioned above. Since the later adjustment with reference to the position of the useful optical surface requires this position to be known down to a few μ, there is the need to provide on the optical unit, for example the mirror, a number of reference surfaces which then form reference surfaces—appropriately referred to a zero point—for installation. A very high economic outlay is required in order to fulfill the requisite accuracy specifications for the reference surfaces. The abovementioned problems are even greater given an optical element deviating from the circular shape, for example a reflecting element or a prism.